The purposes of the formation of a chromate conversion coating on the surface of aluminum are to provide corrosion resistance, improve adhesion of coatings and for aesthetic reasons. The conversion coating improves the adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate conversion coating is typically provided by contacting aluminum with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Typical chromium or chromate conversion coatings exhibit a visible coloration ranging from gold to brown.
Growing concerns exist regarding the pollution effects chromate and phosphate discharged into rivers and water ways by such processes. Because of high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion coating processes require extensive waste treatment procedures to control their discharge.
Attempts have been made to produce chromate free conversion coatings for aluminum. Chromate free pretreatment coatings based upon complex fluoacids and polyacrylic acids are known in the art. U.S. Pat. No. 4,191,596 which issued to Dollman et al., discloses a composition for coating aluminum which comprises a polyacrylic acid and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. U.S. Pat. No. 4,921,552 which issued to Sander et al. discloses a nonchromate coating for aluminum which is dried in place which forms a coating having a weight from about 6 to 25 milligrams per square foot. The aqueous coating composition consists essentially of from more than 8 grams per liter H.sub.2 ZrF.sub.6, more than 10 grams per liter of water soluble acrylic acid and homopolymers thereof and more than 0.17 grams per liter hydrofluoric acid.
U.S. Pat. No. 4,136,073 which issued to Muro et al., discloses a composition and process for the pretreatment of aluminum surfaces using an aqueous acidic bath containing a stable organic film forming polymer and a soluble titanium compound. U.S. Pat. No. 5, 158,622 which issued to Reichgott et al., discloses a dried-in-place conversion coating for metal surfaces such as aluminum and aluminum alloys which employs an aqueous solution of a water soluble maleic or acrylic acid/allyl ether copolymer alone or with an acid.
Most non-chromate based pretreatments generate transparent coatings on metal surfaces. In the use of such pretreatments, the lack of a visible effect such as the color change common in chromate pretreatments makes it virtually impossible to visually verify the presence of a conversion coating. With the prior art chromate pretreatments, it was possible to not only quickly verify visually the presence of the conversion coating, but to also estimate the coating weight by the intensity of the color.
In the treatment of a fast moving coil of metal, visual verification can be very important. With a clear coating, the line must be stopped and a section of metal cut out for testing just to verify the existence of the conversion coating. With the present invention, the color change provides a quick visual verification of the presence of the conversion coating. Prior to the present invention, no methods existed to effectively impart color to a dried-in-place non-chromate polyacrylamide based conversion coating without anodizing the metal surface.